Here comes the third vault, once again different from the previous (1 & 2).

The starting idea is similar to the last one, but materials and function of the manhole are rather different. Whereas the other connects the warehouse to the cellar, this one (smaller) is placed next to the water cistern and its purpose is to make the water supply easier so to spare people the effort to step up the staircase with a heavy bucket.

The construction has been a little troubled. When the vault was almost finished I realized that the whole structure was unrealistic and insecure. So I had to make some drastic changes.
Let's see what I mean.

In these pictures you can see a "resume" of the first construction. Bricks comes like in a normal cross vault, unlike in the middle of it a marble ring is set to frame the round manhole.
At first this solution seems good to me, but something looks strange... I think I've never seen a vault like this and comparing it to the pictures found on the web I finally undersatnd what's wrong.
The stronger part of a cross vault, which allow the structure to self-sustaining when it's finished, is the key stone. Without this central point, the vault would collapse under the vertical strain of the building.

That's what probably'd happen to the vault above.

So how can I set right this situation?
Simply taking a back step and setting the marble ring as the mentioned key. It will support the vault at the time the vault will carry its weight.

To do that I cut off the central square of the vault. Then, on a slightly convex support (a drower knob) I start placing bricks all around the ring following its circumference until I reach the outer frame.

Now I join together the pieces filling the corners with more bricks, and here is my new vault with stone framed manhole. Unless unpredictable disasters, this one will not crumble over my head when I go down to take water...

(If you're thinking that, due to the small scale of the building, the vault would not collapse anyway, you still don't understand a basic feature: when the house will be finished I will really move there. Believe it or not.)